Splitting and samples by chromosome and combining them again in Nextflow

pipeline.nf

chroms = ['chr1', 'chr2', 'chr3'] //let's say we want to try out the first three chromosomes
samples = Channel
    .from (['n1', chroms],
           ['n2', chroms])
           //.subscribe {println it}
           /*
           [n1, [chr1, chr2, chr3]]
           [n2, [chr1, chr2, chr3]]
           */

//Let's split by sample and chromosome
chromosomewise = samples
     .map {it -> [it[0], it[1].collate(1)]}
     //.subscribe {println it}
     /*
     [n1, [[chr1], [chr2], [chr3]]]
     [n2, [[chr1], [chr2], [chr3]]]  
     */
     .transpose()
     //.subscribe {println it}
     /*
     [n1, [chr1]]
     [n1, [chr2]]
     [n1, [chr3]]
     [n2, [chr1]]
     [n2, [chr2]]
     [n2, [chr3]]  
     */
//let's combine again by sample
samplewise = chromosomewise
      .groupTuple()
      //.subscribe {println it}
      /*
      [n1, [[chr1], [chr2], [chr3]]]
      [n2, [[chr1], [chr2], [chr3]]]
      */
      .map{it -> [it[0], it[1].flatten()]}
      //.subscribe{println it}
      /*
      [n1, [chr1, chr2, chr3]]
      [n2, [chr1, chr2, chr3]]
      */